Currently, when a spacecraft, such as a satellite, is moving in an orbit about a planetary body, such as the Earth, the spacecraft encounters disturbances, such as solar wind. These disturbances, left unchecked, will create a series of impulses of momentum which will saturate the momentum accumulator which could cause a loss in attitude. The amount of deviation is directly related to the specific configuration of the spacecraft--the more symmetric a spacecraft is about its center of mass the amount of deviation is minimized. For example, the most severe configuration of satellite with a single reflector can result in the satellite encountering daily momentum accumulations in roll, pitch and yaw by amounts of 5, 10 and 5 Nms, respectively. In a more typical case of a satellite having two symmetrically mounted reflectors, momentum accumulation in roll, pitch and yaw can each amount to 5 Nms.
To counteract such accumulations of momentum, spacecraft employ momentum accumulators which store the momentum encountered by the spacecraft so that the effect of the momentum is minimized or reduced. Examples of well known momentum accumulators is a pyramid of reaction wheels or gimbaled momentum wheels.
These momentum accumulators, however, are unable to accumulate momentum without end. Eventually, the stored momentum needs to be dumped or unloaded during the orbit of the spacecraft. However, the manner in which a spacecraft performs thruster operations to remain in a desired orbit, known as stationkeeping, does have an effect on the momentum dumping capability of the spacecraft. Furthermore, in such stationkeeping, the dumping capability is maximized when the total daily burn time is maximum (maximum inclination delta-v) and when the burn time is distributed most symmetrically throughout the day (minimum eccentricity and longitudinal acceleration delta-v). The worst-case conditions for dumping capability are therefore the minimum-eccentricity control strategy with minimum north-south disturbance in the 17 year life cycle of a satellite and with maximum longitudinal acceleration.
Several stationkeeping methodologies are possible. For example, prior satellites have unloaded momentum simultaneous with North/South stationkeeping only by using thrusters mounted on the North face. Simultaneous control of East/West positioning is not contemplated with this method.
Another possible method of stationkeeping is described in U.S. Pat. No. 5,443,231 to Anzel. That application describes a method of East/West and North/South stationkeeping which uses four gimballed ion thrusters in the same configuration as shown in FIG. 2 of the present application.
In a third example, gimballed ion thrusters are mounted on the North face and the South face of a satellite for momentum unloading during North/South stationkeeping, such as described in U.S. Pat. No. 5,349,532 to Tilley et al. Again, East/West stationkeeping is not performed.
Furthermore, gimballed ion thrusters are used for North-South stationkeeping on the EUROSTAR Spacecraft. This spacecraft appears not to disclose momentum unloading nor the use of the system for East-West stationkeeping.
While the above-mentioned control systems are generally adequate for their intended purpose, there is room for improvement. For example, the fuel efficiency of the above-mentioned control systems is adversely affected because North/South and East/West stationkeeping and momentum dumping are not performed simultaneously. Furthermore, unloading of momentum is not done in an efficient manner by prioritizing which systems are activated to dump momentum.